Process of producing castings of magnesium and high grade magnesium alloys



Patented Jan. 31, 1933 UNITED. STATES PATENT OFFICE" ALBERT LEvY NONI), or LONDON, ENGLAND, ASSIGNOR, BY NEsNE ASSIGNMENTS, 'ro

MAGNESIUM DEVELOPMENT CORPORATION, A CORPORATION OF DELAWARE PROCESS OF PRODUCING CASTINGS OF MAGNESIUM AND HIGH GRADE MAGNESIUM: ALLOYS R0 Drawing. Original application filed December 14, 1929, Serial No. 414,940. Divided and this application filed February 16, 1931, Serial No. 516,219, and \in Great Britain December 19, 1928.

a on the castings incidentally with the casting .processresistant to corrosion to a higher degree than hitherto feasible. Such protective layers are known per se, but their formation required a special treatment, i. e. a distinct operation entirely independent of the casting process. Other objects of the invention will appear fromthe following.

In U. S. Patent 1,463,609, a process of casting magnesium and magnesium alloys is described, which consists in applying to the mould, prior to casting the metal, a substance which is capable of giving ofi, in contact with the molten metal, a gaseous substance, which is more easily oxidizable than V oxalic acid compounds, bicarbonates, urea magnesium. In U. S. Patents 1,584,0 72 and 1,614,820, an improved modification of this process is described, inasmuch as the moulds are prepared from a mixture of green sand with non-metallic substances incapable of reaction with the metal and adapted, when heated by the heat emitted from the 'cast metal, to form on the surface of the casting a layer preventing the casting from being impaired by the access of air and water vapour. Suitable substances for this purpose are mentioned as being sulphur, boric acid,

and mixtures of these substances. It willbe noted that the substances thus claimed are supposed to be incapable of reaction with the metal and it further appears that the protective layer on the metal is only present during the casting' operation, generally in ture with green sand, had iven satisfactory results, and as it is well own that urea under certain conditions is decomposed under the influence of heat into an ammonium salt, namely ammonium isocyanate, with the liberation of ammonia, during casting when using moulding sand contalning urea, and as I have actually observed the presence of ammonia, I tried other ammonium compounds for the same purpose.

It is further known that the resistance to corrosion of articles made of magnesium and magnesium alloys is, by a superficial treatment with fluoride solutions, considerably increased, probably owing to the formation of a dense, although extrem l tenuous, protec tive layer of a magnesiumuorine compound.

With the object of forming a permanent protective layer of this kind, incidentally with the castin process, on the cast articles,-as distinguis ed from the temporary protection afforded by the known substances applied in the casting processes according to the abovementioned patents,I employed an ammonium salt which is combined with fluorine, i. e. ammonium fluoride, in mixture with green sand as a means for creating a neutral atmosphere between the sand and the molten solidifying metal. However, the hygroscopic properties of ammonium fluoride proved to be a great disadvantage, inasmuch as ammonium fluoride in contact with sand and water or moistures, readily decomposes giving ofi ammonia vapours. The moulds thus rapidly become impoverished with respect to the rotective substance and this detrimental e ect is still further promoted by the heating of the sandin the course of casting. The sand therefore practically cannot be used over again without re-additionof ammonium fluoride.

A further object of the present invention is therefore, to avoid the detrimental effects of such hygroscopicity in the material constituting the mould.

This object can be fulfilled in numerous different we s. It may be either achieved embodying 1n the sand, together with ammonium fluoride, an acid which is capable of absorbing ammonia, 'or by employ ng, in

ditlon of this protective ingredient.

stead of ammonium fluoride, a complex chemical compound of said substance having a lesser tendency to decompose under the influence of moisture and heat.

Thus, according to one embodiment of the invention, I employ a fluorine compound which is non-hygroscopic and which at the same time has the property, when coming into contact with the molten metal, of creating a neutral atmosphere and of providing a protective layer on the cast metal. I have found that a suitable substance of this kind isthe acid ammonium fluoride or ammonium hydrofiuoride NHJT. HF, which 15 when mixed with green sand in amounts preferably ranging between 3 and 5 percent ensures perfect castings. As practically no decomposition of the salt occurs the sand may be repeatedly used without further ad- By wa of example I have'furthe'r found that a su stance which in contact with water yields a nonhygroscopic fluoride compound y decomposition, namely ammonium silicofluoride, when admixed to the sand in the; aforementioned proportions, also gives satisfactory results.

The property of this salt of decomposing,

' in the presence of water/to form a non-hygroscopic fluoride offers the advantage that by its employment the free moisture of the green sand is simultaneously reduced, i. e. part of the moisture of the sand is withdrawn. This is probably due to the fact that an equilibrium is established between ammonium silico-fluoride and water, which can be explained by.

the following equation:

40 This property can be proved by the simple experiment of dissolving ammonium silico-fluoride in water, when after a short while silicic acid is precipitated, which is capable of absorbing water. In addition free hydrofluoric acid is formed. When using ammonium silico-fluoride in practice it is advantageous, before forming the mould, to store the sand ,to which the ammonium silico-fluoride has been admixed, preferably in a quantity amounting to about from 3 to 5 percent, for some time, until the reaction has taken place between the ammonium silico-fluoride and the water present in the green sand and until the silica gel formed has absorbed some of the moisture in the green sand.

If this precaution be omitted the sand will swell owing to the increase in volume of the silica gel and if such sand be used immediately the impression of the pattern will become distorted. The free hydrofluoric acid or other fluorine compounds that may be pro duced during casting have the property of forming an anti-corrosive layer on the metal 65 itself.

- According to a. modification of the inven-- to a' sand containing. the hygroscopic :am-

monium fluoride, very efliciently reduces the decomposition of the latter in contact with the moisture of the sand. Thus a green sand thoroughly mixed with about 5 percent of ammonium fluoride and about 3 percent of oxalic acid shows hardly any decomposition and is suitable for casting magnesium and. magnesium allo s, at the same time provid-' castings with a protective ing the finishe layer of a fluorine compound.

I have further found that the tendency of the ammonium si-lico-fluoride to cause a swelling of the moist moulding sand, thus necessitating a storing of the sand before using it for moulding, is also substantially checked by incorporating, in the sand, oxalic acid, preferablyalso in a quantity of about 3 percent, together with ammonium silico-fluoride. In this case, a storing of the sand before use is no longer necessary.

The substances and compositions hereinbefore described, may in the case of small castings be employed by themselves in amounts preferably not exceeding 15 percent. In the case of larger castings, however, it is frequently advisable to further add to the sand one or more of the protective substances disclosed in the specifications of the U. S. Patents 1,463,609, 1,584,072, and 1,614,820, preferably small percentages of sulphur and/or boric acid, so as to efliciently prevent local scorchings.

, It will be understood that the application of the substances hereinbefore described is not limited to impregnating the sand by admixing these substances, although this is generally the most economical and simple way of applying them. Any mode of applicatlon which will ensure contact between the molten metal and the protective substances disclosed is also suflicient to impregnate the sand constituting the inner surface of the finished mould with the substances named before casting the metal into the mould. e

. I claim: v

1. The method of casting magneslum and material ammonium silco-fluoride, forminga 'within the mould falls within the scope of 1 the present invention. Thus, for example, it

mould of the mixture and casting the metal material ammonium silico-fluoride and oxalic acid, forming a mould of the mixture, and casting the metal in the mould.

3. The method of casting magnesium and high grade magnesium alloys in sand moulds, comprising incorporating in the moulding material ammonium silico-fluoride and sulphur, forming a mould of the mixture, and casting the. metal in the mould.

4. The method of casting magnesium and high grade magnesium alloys in sand moulds, comprising incorporating in the moulding material ammonium silico-fluoride, storing the mixture until swelling no longer occurs, forming a mould of the mixture and casting the metal in the mould.

In testimony whereof I have hereunto set my hand.

ALBERT MOND. 

